In the TW-EDM art, attempts to obtain higher removal rate with due machining quality and economy, give rise to various problems vis a vis the wire electrode. The wire must be capable of carrying electrical discharge current with higher density to achieve higher rate of erosion and yet need be thinner (typically 0.05 to 0.5 mm in thickness) to assure higher cutting accuracy. To allow the wire and hence the erosion to faithfully follow the programmed machining path, a high tension must be applied to the wire to maintain its active electrode length traveling through the cutting zone as straight or geometrically accurate as possible. Since the wire purchased is consumed or "used up", it is economically desirable that it be "used up", that is, advanced to renew the active electrode surface through the cutting zone as slowly as possible. Since the wire is thin, the cutting groove created behind the traversing wire in the workpiece is correspondingly narrow so that machining discharge products tend to accumulate in the cutting zone through which the wire travels. This tendency increases as the thickness of the workpiece or the active electrode length of the wire increases. All these factors and requirements severely affect the ability of the wire to withstand breakage. In TW-EDM, the wire breakage is fatal since it interrupts the machining process. Therefore, higher removal rate cannot be pursued without overcoming higher risk of wire breakage which arise from the use of greater energy and higher repetition rate of machining discharge pulses as required to increase the erosion rate.
In TW-EDM, machining is considered to proceed in a normal mode when electrical discharges occur uniformly throughout the active electrode surface of the wire traveling through the cutting zone. Under an adequate set of operating parameters, the wire will not then break if machining is allowed to proceed at a maximum efficiency by permitting pulses applied to result in such random electrical discharges at a highest proportion and/or the wire to travel at a minimum speed. The wire passed out of the cutting zone can then be considered to have been truly "used up" upon removing stock from the workpiece at maximum efficiency. As long as machining proceeds in the normal mode, the pulses and the contouring feed can be enhanced to increase removal rate to a maximum level which the eventual set of operating parameters allows.
In a TW-EDM operation, however, it is practically not possible to exclude the possibility that such a normal machining mode is disturbed. Machining is more likely disturbed as higher removal is attempted to achieve by employing an "enhanced" machining condition where machining conditions are relatively "low" or safer against wire breakage to allow a relatively low removal rate, machining if disturbed is more likely returned spontaneously to the normal mode. Machining is considered to become disturbed if a uniform distribution of successive electrical discharges over the entire active electrode surface of the wire traveling through the cutting zone is impared. If this condition continues or is allowed to continue, discharges tend to become "abnormal" in the cutting zone. The random electrical discharges will progressively be reduced in number and concentrate at particular sites in the cutting zone. Eventually the wire will be broken at a certain point where its strength is the weakest or it can no longer withstand excessive thermal and mechanical stresses due to the localized abnormal electrical discharges.
While it has thus been generally recognized that wire breakage is of major concern and is in effect triggered by a disturbance of the normal machining mode and an abnormal concentration of electrical discharges, difficulties have been encountered in the prior art in timely detecting a real tendency for the wire to break or properly predicting a wire breakage during the machining process. It has been found that electrical detection of discharges themselves most often does not provide an adequate, valid and reliable information to this end. Accordingly, given a particular electrode wire, it has been customary that one is compelled to choose safer machining conditions with respect to the possibility of its breakage, or must be satisfied with a removal rate which is much lower than that which ought to be readily obtainable if the wire does not break.